The invention relates to a method and device for reducing register errors in multicolor rotary printing machines and more particularly, in multicolor offset printing machines having printing units driven by a common motor and having a register-adjusting device.
The production of satisfactory multicolor prints presupposes that the printing plates used for printing the different colors are in exact registry with one another i.e. they coincide or match up with great accuracy in the paper travelling through the printing machine. In order to achieve this exact coincidence, such printing machines are provided with register-adjusting devices which permit mutual adjustment about small angular values of the respective cylinders carrying the printing plates in the consecutive printing units.
A particular difficulty encountered in register adjustment is that the angular position of the cylinders carrying the printing plates is not rigid or fixed with respect to the drive shaft but rather, because of the unavoidable elasticity of the materials used, is dependent upon the torque which must be applied for driving the printing machine. This torque is greatest at the drive motor, and decreases therefrom along the drive chain extending to the individual printing units. Consequently, the torque-dependent angular displacement of the cylinder carrying the printing plates is different from that of the drive shaft, from which it also follows that the angular displacement of the cylinders carrying the printing plates is different from one another and is dependent upon the magnitude of the torque. Because, on the other hand, the torque to be applied by the drive motor is a function of the operating speed of the printing machine, an exact register adjustment can be effected only at the desired printing rate, which results in the production of considerable waste of paper before the correct register setting is attained, when the printing machine is being set up at high speeds.
To remedy this deficiency, automatic register-adjustment devices have been provided heretofore which include machine-readable markings on the cylinders carrying the printing plates, as well as sensors which are responsive to these markings and deliver signals to a control unit which enable the control unit to ensure good coincidence or matching of the printed images through suitable register adjustment. Such a device is very costly, however, and is, especially disadvantageously, not readily applicable for retrofitting on existing printing machines.
It is accordingly an object of the invention to provide a method and a device for preventing register errors in multicolor rotary printing machines which, after precise adjustment or setting of the registry at any desired and especially low speed, permits the operating speed of the printing machine to be changed without again losing the registry setting.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for reducing register errors in multicolor offset printing machines having printing units driven by a common motor and having a register adjusting device, which includes determining and storing a functional relationship between a quantity selected from a group of quantities consisting of the quantity of torque delivered by the common motor and a quantity characteristic of the torque, on the one hand, and a register adjustment necessary for maintaining satisfactory register, on the other hand, monitoring the quantity during operation of the printing machine, and setting the register adjustment to a value functionally associated with the respective torque.
Because the torque-dependent angular displacement of the plate cylinders with respect to the drive shaft is dependent upon the mechanical construction and especially, upon the elasticity characteristics of the materials which are used, a specific characteristic of the respective printing machine is involved which is not subject to any appreciable variations with respect to time. Therefore, while the printing machine is operating slowly, the pressman can set up a satisfactory registry or coincidence of the individual printed images, and this registry or coincidence is maintained even if the machine is brought up to high speed, because the register adjustment necessary for maintaining the registry or coincidence is known and is performed automatically. Likewise, the invention prevents register errors which might otherwise occur due to fluctuations in operating conditions, for example due to a rise in the operating temperature with consequent reduction in the torque-requirement with increasing operating or running time after start-up of the printing machine. The method according to the invention, therefore, permits the printing machine to be set up at low speed with correspondingly low consumption or wastage of material, and ensures the maintenance of good registry at the desired printing rate even under changing operating conditions, without requiring recourse to be taken to heretofore known, complicated control equipment. On the contrary, a relatively simple control based upon the once-determined functional relationship between the torque delivered by the motor and the necessary register adjustment is sufficient.
Of particular advantage is that a device for implementing the method according to the invention can be installed on each printing machine, even retrofitted on already existing machines, because it is merely necessary to install a device for monitoring the torque delivered by the motor or a quantity characteristic of the torque and to connect it to a control device having a memory wherein register adjustment values are stored which are associated with different torques, the control device being connected to the register-adjustment device of the printing machine in such a manner that the control device, depending upon the measured torque, effects a setting of the register to the associated, stored value.
Therefore, in accordance with another aspect of the invention, there is provided a device for reducing register errors in multi-color offset printing machines having printing units driven by a common motor and having a register-adjusting device, including means for monitoring a quanity selected from a group of quantities consisting of the quantity of torque delivered by the common motor and a quantity characteristic of the torque, and a control unit including a memory with register adjustment values associated with different torques stored therein, the control unit being actuatable for setting the register adjustment to the respective associated stored value in accordance with the monitored selected quantity. In this regard, it is believed to be readily apparent that, in offset printing machines, besides the cylinders carrying the printing plates, the blanket cylinders may also be adjusted in order to prevent image displacement on the rubber blanket and consequent spoiled prints.
In accordance with another mode of the invention, the method includes measuring the torque at the drive shaft. To accomplish this, the application of resistance strain gauges may be sufficient. It is also possible, however, to install, for example, piezoelectric torque sensors in the drive shaft of the motor. It is even simpler, in accordance with an alternative mode of the invention, to provide a method which comprises measuring the current consumption of the common motor in order to determine the torque. Even if the functional relationship between torque and current consumption or other operating variables of the motor is not linear, this is of no special importance if, as in accordance with other alternate modes of the invention, the method comprises determining the functional relationship either by at least one trial run or by computation or calculation.
With regard to a practical implementation of the method according to the invention, in accordance with yet an additional mode of the invention, the method includes storing the functional relationship between the torque or a quantity characteristic thereof and the register adjustment in the form of a table of values, because the data obtained in the table of values can then be processed by conventional computation devices, especially a digital processor.
In principle, the values corresponding to different register adjustments are then stored at addresses corresponding to different torques so that the measuring of a specific torque and a quantity characteristic thereof, respectively, results in the addressing of a specific memory location, while the value present at the memory location determines the register adjustment associated with the torque. The transfer of this value into the desired register adjustment can be effected readily with conventional control means. Likewise, it is apparent that it is readily possible to convert the measured torque into a digital value corresponding directly to a memory address.
Although the functional relationship between torque and register adjustment is a characteristic of the respective printing machine, this characteristic may also depend upon external factors, for example the temperature, since the elasticity of the materials is also temperature-dependent. Furthermore, the register adjustment depends upon the manner of distributing the loading over the device chain, so that the use of inks of different viscosity may also have an effect upon register adjustment. Furthermore, the register adjustment depends upon the material which is used as the printing carrier, thus for example upon the composition and thickness of the paper used, and perhaps also on film or plastic material or even pieces of textile material. Accordingly, the invention also includes a method which comprises determining the functional relationship between the torque or a quantity characteristic of the torque and the register adjustment for different conditions, such as different temperatures, using inks of different viscosities and/or using different materials as print carriers, and controlling the register adjustment for each of the different conditions prevailing. Such an influencing condition may be, for example, the shutting-down of one or more printing units, or general shifts in the torque due to changes of functions.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for reducing register errors in multi-color rotary printing machines, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.